Cardiac arrhythmias, and atrial fibrillation in particular, persist as common and dangerous medical ailments, especially in the aging population. In patients with normal sinus rhythm, the heart, which is comprised of atrial, ventricular, and excitatory conduction tissue, is electrically excited to beat in a synchronous, patterned fashion. In patients with cardiac arrhythmias, abnormal regions of cardiac tissue do not follow the synchronous beating cycle associated with normally conductive tissue as in patients with normal sinus rhythm. Instead, the abnormal regions of cardiac tissue aberrantly conduct to adjacent tissue, thereby disrupting the cardiac cycle into an asynchronous cardiac rhythm. Such abnormal conduction has been previously known to occur at various regions of the heart, such as, for example, in the region of the sino-atrial (SA) node, along the conduction pathways of the atrioventricular (AV) node and the Bundle of His, or in the cardiac muscle tissue forming the walls of the ventricular and atrial cardiac chambers.
Cardiac arrhythmias, including atrial arrhythmias, may be of a multiwavelet reentrant type, characterized by multiple asynchronous loops of electrical impulses that are scattered about the atrial chamber and are often self propagating. Alternatively, or in addition to the multiwavelet reentrant type, cardiac arrhythmias may also have a focal origin, such as when an isolated region of tissue in an atrium fires autonomously in a rapid, repetitive fashion.
Several pharmacological approaches intended to remedy or otherwise treat atrial arrhythmias have been disclosed, although such pharmacological solutions are not generally believed to be entirely effective in many cases, and may in some cases result in proarrhythmia and long term inefficacy. Several surgical approaches have also been developed with the intention of treating atrial fibrillation. One particular example is known as the “maze procedure.” In general, the maze procedure is designed to relieve atrial arrhythmia by restoring effective atrial systole and sinus node control through a prescribed pattern of incisions about the tissue wall. In the early clinical experiences reported, the maze procedure included surgical incisions in both the right and the left atrial chamber. However, more recent reports predict that the surgical maze procedure may be substantially efficacious when performed only in the left atrium.
Success with surgical interventions through atrial segmentation, particularly with regard to the surgical Maze procedure, has inspired the development of less invasive catheter-based approaches to treat atrial fibrillation through cardiac tissue ablation. Examples of such catheter-based devices and treatment methods have generally targeted atrial segmentation with ablation catheter devices and methods adapted to form linear or curvilinear lesions in the wall tissue which defines the atrial chambers, such as those disclosed in U.S. Pat. No. 5,617,854 to Munsif, U.S. Pat. No. 4,898,591 to Jang, et al., U.S. Pat. No. 5,487,385 to Avitall, and U.S. Pat. No. 5,582,609 to Swanson, the disclosures of which are incorporated herein by reference. The use of particular guiding sheath designs for use in ablation procedures in both the right and left atrial chambers are disclosed in U.S. Pat. Nos. 5,497,774, 5,497,119, 5,564,440 and 5,575,766 to Swartz et al., the disclosures of which are incorporated herein by reference. In addition, various energy delivery modalities have been disclosed for forming such atrial wall lesions, and include use of microwave, laser and more commonly, radiofrequency energies to create conduction blocks along the cardiac tissue wall, as disclosed in International Publication No. WO 93/20767 to Stern, et al., U.S. Pat. No. 5,104,393 to Isner, et al. and U.S. Pat. No. 5,575,766 to Swartz, et al., respectively, the disclosures of which are incorporated herein by reference.
Ablation is most effectively performed when the distal tip electrode is in contact with the cardiac wall. Absence of contact or poor contact of the tip electrode with the heart wall leads to dissipation of the RF energy in the blood, as well as possible fouling of the tip electrode with the concomitant possibility of blood clot formation. Accordingly, it is important that both mapping and ablation be accompanied by methods and systems for detecting and ensuring electrode-tissue contact. A number of references have reported methods to determine electrode-tissue contact, including U.S. Pat. Nos. 5,935,079; 5,891,095; 5,836,990; 5,836,874; 5,673,704; 5,662,108; 5,469,857; 5,447,529; 5,341,807; 5,078,714; and Canadian Patent Application 2,285,342. A number of these references, e.g., U.S. Pat. Nos. 5,935,079, 5,836,990, 5,447,529, and 6,569,160 determine electrode-tissue contact by measuring the impedance between the tip electrode and a return electrode. As disclosed in the '160 patent, the entire disclosure of which is hereby incorporated by reference, it is generally known that impedance through blood is generally lower that impedance through tissue. Accordingly, tissue contact has been detected by comparing the impedance values across a set of electrodes to pre-measured impedance values when an electrode is known to be in contact with tissue and when it is known to be in contact only with blood.
The success of catheter based ablation procedures has led to numerous improvements to the catheters used for the procedures. However, the traumatic nature of the ablation procedure has given rise to certain complications. One such complication is the possibility of damaging the esophagus, which lies very close to, and often touches the outer wall of the left atrium. Damage to the esophagus is sometimes caused when the esophagus touches or is close to the tissue in the left atrium that is being ablated. The heat from the ablation procedure may penetrate through the tissue of the left atrium and reach the esophagus. This damage to the esophagus is extremely dangerous, as the damaged esophagus often becomes infected. Due to this infection, an esophageal fistula, or hole in the esophagus, develops over time, causing the infection to spread to the heart wall. This damage to the esophagus carries an extremely high mortality rate.
To avoid damage to the esophagus, a need exists for a method of locating the esophagus during catheter-based procedures within the heart, such as mapping and/or ablation. To that end, some physicians have used standard mapping catheters to record the pre-procedure location of the esophagus. However, such a pre-procedure location determination fails to account for the mobile nature of the esophagus. The esophagus generally does not remain stationary. Rather, the esophagus often moves back and forth such as when the patient swallows or coughs, thereby positioning itself in different locations relative to the heart wall. As such, the esophagus may change its location during a catheter-based endocardial procedure. The pre-procedure determination fails to account for this movement. Accordingly, a need exists for a method of locating the esophagus during mapping and/or ablation procedures.